Biometric authentication in which features of biometric information are used to identify individuals has lately been attracting attention. Examples of biometric information include fingerprints and iris or blood vessel patterns.
Advantages of biometric authentication are convenience and high security, which are owing to the fact that biometric authentication does not require a person to carry a key with him/her and has less fear of fraudulent acts as a consequence of the loss or theft of the key, or the like.
Of the varying types of biometric authentication, authentication using blood vessel patterns (blood vessel authentication) is becoming popular. Blood vessel patterns which are information within a living body are more difficult to counterfeit than fingerprints. The security of blood vessel authentication is accordingly higher than that of fingerprint authentication. Further, in blood vessel authentication an eyeball does not need to be irradiated with light, unlike iris authentication, which helps users to feel less reluctant to use blood vessel authentication and makes blood vessel authentication safe to human health.
An example of a blood vessel authentication device is disclosed in JP 07-21373 A. The blood vessel authentication device irradiates a human body with near-infrared light. Using an image sensor, the blood vessel authentication device takes a photograph with transmitted light and reflected light. Hemoglobin in blood absorbs near-infrared light more than the rest of the body does, and hence a blood vessel pattern is extracted in the picked up image. The blood vessel authentication device determines whether or not the blood vessel pattern extracted in the picked up image matches any blood vessel pattern registered in advance. Based on the result of the blood vessel pattern matching, the blood vessel authentication device identifies an individual.
In the case where a biometric authentication device is mounted to a portable electronic device such as a cellular phone, it is difficult to secure enough space to mount a biometric authentication device in the portable electronic device. Reduction in size of biometric authentication devices is therefore necessary.
For instance, a normal fingerprint authentication device includes a contact detection type optical image sensor, a pressure-sensitive sensor, or the like as a sensor for measuring a fingerprint pattern. These sensors have approximately the same size as the size of a human fingertip, and are difficult to mount in a mobile electronic device. As a solution, a fingerprint authentication device equipped with a small line sensor has been developed. This fingerprint authentication device photographs a finger sliding over the small line sensor. A problem of the fingerprint authentication device is consequently the poor quality of the obtained fingerprint image.
Blood vessel authentication devices, on the other hand, include an image sensor such as a charge coupled device (CCD) sensor or a complementary metal-oxide semiconductor (CMOS) sensor. These image sensors can be reduced in size, but a separate lens is necessary for forming an image on the image sensor. Reducing the overall size of a blood vessel authentication device is therefore not easy.
JP 2005-346238 A discloses a fingerprint authentication device including a translucent image sensor overlaid on a liquid crystal display. With this structure, an image sensor can be disposed over a liquid crystal display of a cellular phone and therefore does not require a space dedicated to the image sensor. Further, a backlight for the liquid crystal display can be utilized, without any modifications, as an irradiation light source for photographing.
Known technologies are employable in manufacturing the translucent image sensor. According to the known technologies, a sensor array is formed by laminating an amorphous silicon layer or a polysilicon layer on a glass substrate.